Catalyst, its preparation and the polymerization of cyclic ethers over this catalyst

ABSTRACT

The present invention relates to a solid, acid catalyst for the preparation of polytetrahydrofuran, polytetrahydrofuran copolymers, diesters or monoesters of these polymers by polymerization of tetrahydrofuran in the presence of at least one telogen and/or comonomer, which has a BET surface area of at least 160 m 2 /g and an acid center density of at least 0.05 mmol/g for pK a  values of from 1 to 6, to a process for preparing it and to a process for the polymerization of cyclic ethers over this catalyst.

CROSS REFERENCE TO RELATED APPLICATION

The present application is a National Stage application ofPCT/EP2003/010149, filed Sep. 12, 2003, which claims priority fromGerman Patent Application No. DE 102 45 198.2, filed Sep. 27, 2002.

The present invention relates to an improved solid, acid catalyst forthe ring-opening polymerization of cyclic ethers. Furthermore, thepresent invention relates to a process for preparing polytetrahydrofuranor polytetrahydrofuran derivatives over such catalysts and to a processfor preparing such catalysts by acid activation.

Polytetrahydrofuran (hereinafter referred to as “PTHF”), also known aspolyoxybutylene glycol, is a versatile intermediate in the plastics andsynthetic fibers industry and is used, inter alia, as diol component forpreparing polyurethane, polyester and polyamide elastomers. In addition,it is, like some of its derivatives, a valuable auxiliary in manyapplications, e.g. as dispersant or in the deinking of waste paper.

PTHF is usually prepared industrially by polymerization oftetrahydrofuran (hereinafter referred to as “THF”) over suitablecatalysts in the presence of reagents whose addition makes it possibleto control the chain length of the polymer chains and thus set the meanmolecular weight (chain termination reagents or “telogens”). The controlis achieved by the choice of type and amount of the telogen. Whenappropriate telogens are selected, functional groups can additionally beintroduced at one end or both ends of the polymer chain.

Thus, for example, the monoesters or diesters of PTHF can be prepared byusing carboxylic acids or carboxylic anhydrides as telogens. PTHF itselfis formed only by subsequent saponification or transesterification. Thispreparation is therefore referred to as a two-stage PTHF process.

Other telogens act not only as chain termination reagents, but are alsoincorporated into the growing polymer chain of PTHF. They not only havethe function of a telogen but at the same time act as a comonomer andcan therefore be referred to as telogens and as comonomers with equaljustification. Examples of such comonomers are telogens having twohydroxy groups, for example dialcohols. These may be, for example,ethylene glycol, propylene glycol, butylene glycol, 1,3-propanediol,1,4-butanediol, 2-butyne-1,4-diol, 2,2-dimethyl-1,3-propanediol,1,6-hexanediol or low molecular weight PTHF.

Further suitable comonomers are cyclic ethers, preferably three-, four-and five-membered rings, e.g. 1,2-alkylene oxides, e.g. ethylene oxideor propylene oxide, oxetane, substituted oxetanes such as3,3-dimethyloxetane, and THF derivatives such as3-methyltetrahydrofuran, 3,3-dimethyltetrahydrofuran or3,4-dimethyltetrahydrofuran.

The use of such comonomers or telogens leads, with the exception ofwater, 1,4-butanediol and low molecular weight PTHF, to the preparationof tetrahydrofuran copolymers, hereinafter referred to as THFcopolymers, and in this way makes it possible to achieve chemicalmodification of PTHF.

Industrially, PTHF can be produced in a single stage by polymerisationof THF using water, 1,4-butanediol or low molecular weight PTHF astelogen over acid catalysts. Known catalysts include both homogeneoussystems dissolved in the reaction system and heterogeneous, i.e. largelyundissolved systems. However, a disadvantage is the relatively low THFconversions which are achieved, especially in the synthesis of PTHFhaving a molecular weight of from 650 to 3000.

On a large industrial scale, use is made predominantly of theabovementioned two-stage processes in which THF is firstly polymerized,e.g. in the presence of fluorosulfonic acid, to form polytetrahydrofuranesters and these are subsequently hydrolyzed to PTHF. This form of theTHF polymerization usually achieves higher THF conversions than in thecase of single-stage processes. Polymerization of THF in the presence ofcarboxylic anhydrides, e.g acetic anhydride, in the presence of acidcatalysts to form PTHF diacetates and subsequent transesterification ofthe PTHF diacetates with, for example, methanol to give PTHF and methylacetate is particularly advantageous.

The preparation of PTHF by polymerization of THF in the presence ofcarboxylic anhydrides or the preparation of THF copolymers bypolymerization of THF in the presence of carboxylic anhydrides andcyclic ethers as comonomers over solid acid catalysts, as is preferredin the present patent application, is known.

DE-A-28 01 578 describes a process for preparing PTHF diacetates fromTHF in the presence of carboxylic anhydrides and a bleaching earthhaving a water content of <3% by weight as catalyst.

DE-A-198 01 462 describes acid-activated calcium montmorillonites havinga specific surface area of at least 300 m²/g, an acidity of at least0.02 mmol/g for pK_(a) values of <−3 and pore volumes of at least 0.4cm³/g for pore sizes in the range 30-200 A as catalysts in powder orextrudate form for the polymerization of THF to give, inter alia, PTHFdiacetates.

U.S. Pat. No. 4,228,462 describes a method of preparing copolymers ofTHF and alkylene oxides over acid-activated montmorillonites having porevolumes of 0.4-0.8 cm³/g, average pore sizes in the range 0.1-0.3 μm anda surface area of 220-260 m²/g.

In DE-A1-197 554 15, PTHF is prepared over a catalyst having a highproportion of montmorillonite. To prepare the catalyst, montmorilloniteis exposed to an acid having a concentration of 2-100% for 0.5-24 hoursat a temperature of 30-120° C. The catalyst is dried at 80-200° C. andcalcined at 150-600° C. and has a BET surface area of at least 150 m2/g.

U.S. Pat. No. 6,274,527 discloses a catalyst based on acid-activatedAlgerian bentonites. The specific raw clay is activated by means ofsulfuric acid having a concentration of 0.1-0.9 mol/l for up to 3 daysat room temperature or 1-2 hours at elevated temperature, filtered,washed and dried.

U.S. Pat. No. 4,127,513 describes the preparation of PTHF copolymersover acid-activated montmorillonites which have an acid center densityof 0.1-0.9 mmol/g at acid strengths (pK_(a)) in the range from −3 to −8.The raw clay is reacted with acids having a concentration of 15-40%,preferably at room temperature, to obtain the specific acidityproperties.

Processes for preparing acid-activated clay minerals, in particularsheet silicates, are likewise known; an overview is given in EP-B 398636 and the publications cited therein. Acid-activated bentonites areused on a large scale as bleaching earths for decolorizing oils.

The catalysts of the prior art display only a relatively low activity,which in an industrial plant leads to very large reactor volumes and/orvery long reaction times. The economics of a heterogeneously catalyzedprocess for the polymerization of cyclic ethers, in particular THF,therefore depends critically on the productivity of the catalyst. It isan object of the present invention to provide a highly active catalystfor the preparation of polytetrahydrofuran, polytetrahydrofurancopolymers, diesters or monoesters of these polymers.

We have found that this object is achieved by a solid, acid catalyst forthe preparation of polytetrahydrofuran, polytetrahydrofuran copolymers,diesters or monoesters of these polymers by polymerization oftetrahydrofuran in the presence of at least one telogen and/orcomonomer, which has a BET surface area of at least 160 m²/g, preferablyat least 220 m²/g and particularly preferably at least 260 m²/g, and anacid center density of at least 0.05 mmol/g, preferably at least 0.15mmol/g, particularly preferably at least 0.25 mmol/g, for pK_(a) valuesof from 1 to 6, preferably in the range from 2.5 to 5.5.

The catalyst comprises a total amount of alkali-soluble andnon-alkali-soluble SiO₂ of at least 20% by weight, preferably at least30% by weight, in particular from 50 to 90% by weight, and at least onefurther oxide of an element selected from the group consisting of Al, Feand the elements of groups IIIA to VIIA of the Periodic Table (groupsdesignated in accordance with the old IUPAC nomenclature), preferablyselected from among the group consisting of Al, Fe, Ti and Zr.Furthermore, the catalyst has a proportion of alkali-soluble SiO₂(amorphous silica) of from 20 to 85% by weight, preferably from 25 to75% by weight, in particular from 30 to 65% by weight.

Furthermore, the catalyst has a pore volume (BJH N₂ isotherms) of atleast 0.2 cm³/g, preferably at least 0.35 cm³/g and particularlypreferably at least 0.45 cm³/g, for pore diameters in the range from 2to 200 nm. The pore volume made up by pores having diameters in therange from 5 to 50 nm is at least 0.1 cm³/g, preferably at least 0.2cm³/g, particularly preferably at least 0.25 cm³/g. The mean BJH porediameter (4V/A) for pore sizes of from 2 to 200 nm is from 2.0 to 10.0nm, preferably from 3.5 to 9 nm.

As raw material for the catalyst of the present invention, preference isgiven to using clay minerals of the montmorillonite/saponite group orPalygorskite/sepiolite group, particularly preferably montmorillonitesas described, for example, in Klockmanns Lehrbuch der Mineralogie,16^(th) edition, F. Euke Verlag 1978, pages 739-765.Montmorillonite-containing minerals are also referred to as bentonitesor occasionally as Fuller's earths.

Suitable clay mineral sources are in principle allmontmorillonite-containing deposits as are listed in, for example, themonograph “The Economics of Bentonite”, 8^(th) edition 1997, RoskillInformation Services Ltd, London. The raw clays frequently comprisemontmorillonite together with further mineral and nonmineralconstituents. As mineral constituents, it is possible for, for example,quartz, feldspar, kaolin, muscovite, zeolites and titanium oxides, ironoxides, calcite and/or gypsum to be present in varying amounts.Preferred raw materials are those having a high montmorillonite contentand a correspondingly low content of secondary constituents. Themontmorillonite content can, for example, be determined via thedetermination of the methylene blue adsorption.

Preferred raw materials have a methylene blue value of at least 250mg/g, preferably at least 290 mg/g, in particular at least 320 mg/g.Particularly preferred raw materials are those whose exchangeablecations are made up to a high percentage of alkali metals, in particularsodium. Based on charge equivalents, these raw materials contain atleast 25%, preferably at least 40%, of monovalent exchangeable cations.These sodium bentonites as raw materials occur in nature; known sourcesof sodium-containing bentonites are located, for example, in Wyoming/USAor in India, and are also known on the basis of their origin as “Westernbentonites” or “Wyoming bentonites” or on the basis of their propertiesas “swelling bentonites”. Bentonites having a high proportion ofalkaline earth metal cations, in particular calcium, are known, forexample, as “subbentonites” or “Southern bentonites” and can beconverted into sodium-containing bentonites by alkaline activation. Suchalkali-activated raw materials are also suitable for the catalysts ofthe present invention. Finally, it is in principle also possible toprepare such raw materials synthetically.

Clay minerals of natural origin occasionally contain nonmineralimpurities, in particular carbon compounds. As catalyst raw material,preference is given to those bentonites which have a total carboncontent of less than 3% by weight, preferably less than 1% by weight,particularly preferably less than 0.5% by weight.

According to the present invention, the catalyst is prepared bysubjecting the clay mineral, either in pelletized, lump or powder formto acid activation. During this activation, at least 50%, preferably atleast 60% and particularly preferably at least 70%, of the acid used isreacted. Based on the catalyst raw material, from 3 to 15milliequivalents, preferably from 4.5 to 12 milliequivalents,particularly preferably from 5 to 10 milliequivalents, of acid arereated per gram (meq/g) of raw clay used. For the purposes of thepresent invention, an equivalent (eq) of a substance is that amountwhich, in a defined reaction, combines with or liberates or replacesthat amount of hydrogen which is bound to 3 g of carbon in ¹²CH₄. Thesefigures are based on a dried raw clay having a high purity, i.e. havinga loss on ignition of 15-20% and a methylene blue value of 300-400 mg/g.In the case of moister or drier raw clays or a lower purity, for exampledue to the presence of extraneous minerals, the amount of acid used andreacted per amount of clay has to be modified appropriately. The optimumamount of acid reacted in the activation is preferably determined forthe particular raw material by means of preliminary tests.

The activation can be carried out using inorganic or organic acids.Preference is given to using hydrochloric acid and/or sulfuric acid ormixtures of hydrochoric acid and/or sulfuric acid with other inorganicor organic acids.

The proportion of raw clay in the activation suspension is from 2 to 50%by weight, preferably from 5 to 40% by weight and particularlypreferably from 10 to 30% by weight. Relatively high crude clayconcentrations are economically advantageous; the upper limit is imposedby the viscosity of the activation suspension.

In the case of the preferred use of sulfuric acid for the activation,the acid conversions in the activation are from 50 to 90%, preferablyfrom 60 to 80%; in the case of the likewise preferred use ofhydrochloric acid for the activation, the conversions are from 70 to100%, preferably from 85 to 95%.

The total amount of acid to be used can easily be calculated from theproportion of raw clay in the activation suspension and the desiredpercentage and absolute acid conversion. The acid used for theactivation can be placed in the reaction vessel at the beginning of theactivation or can be added; it is preferably added in a plurality ofsteps at the beginning and during the activation. As a result of thepreferred addition of the acid in a plurality of steps, the acid ispresent in a lower concentration at the start of the activation thanwould be the case if the total amount of acid were added at thebeginning of the activation. The concentration of acid at the beginningof the activation is less than 4 eq/kg of suspension, preferably lessthan 2.5 eq/kg of suspension and particularly preferably from 0.5 to 1.5eq/kg of suspension.

A critical factor for the selection of the combination of temperatureand time for successfully carrying out the process of the presentinvention is the achievement of the acid conversions specified accordingto the present invention. Preference is given to using temperatures ofat least 60° C. at activation times of at least 4 hours, in particulartemperatures of at least 70° C. at activation times of at least 24 hoursand particularly preferably temperatures of at least 80° C. atactivation times of at least 36 hours.

After the acid activation is complete, the catalyst is separated offfrom the mother liquor and freed of adhering acid residues using methodsknown to those skilled in the art. For this purpose, the activationsuspension can firstly be separated from the major part of the motherliquor by, for example, filtration, centrifugation or sedimentation anddecantation, with preference being given to filtration andcentrifugation. The catalyst is subsequently freed of the remainingmother liquor and adhering acid residues by washing with a solvent,preferably demineralized water. The washing process can be carried outcontinuously, for example in filter presses, or batchwise, for exampleby repeated suspension/decantation. The washing medium can be given aslightly acidic pH by small additions of acid in order to reduceadsorption of basic trace components from the washing medium on thecatalyst. Preference is given to using aqueous nitric acid having a pHof 2-5 for this purpose. The washing times can be shortened by using thesolvent at a higher temperature. Washing medium having a low level ofcontamination obtained toward the end of the washing process can bereused at the commencement of washing of a further batch.

After washing, the catalyst preferably contains little if any free acid,i.e. little if any residues of the acid used for the activation remainin the catalyst. It is preferred that less than 20%, particularlypreferably less than 5%, of the total acid centers are present in theform of free acid on the catalyst. The proportion of free acid on thecatalyst can be reduced to values according to the present invention bymeans of the washing process.

After separating off and washing out the mother liquor, the catalyst isdried, if desired at elevated temperature and/or reduced pressure and/orby passing dry gases such as air or nitrogen through it and/or byslurrying and spray drying it. The catalyst is preferably dried at from30° C. to 200° C., under atmospheric or subatmospheric pressure. Thecatalyst can subsequently be calcined at from 150° C. to 800° C.,preferably from 250° C. to 600° C.

The catalyst can be used in the polymerization reaction in powder formor preferably as shaped bodies. The production of shaped bodies frompulverulent raw materials can be carried out by methods known to thoseskilled in the art, for example tableting, agglomeration or extrusion,as are described, inter alia, in the Handbook of Heterogenous Catalysis,Vol. 1, VCH Verlagsgesellschaft Weinheim, 1997, pp. 414-417. In theshaping procedure, auxiliaries known to those skilled in the art, e.g.binders, lubricants and/or solvents, can be added. It is equallypossible to shape the raw clay and activate it as shaped bodies or tocarry out an activation of powders and shape the activated powder.Finally, it is also possible to activate raw material in the form oflumps or pieces and to use it as catalyst in this form. The acidactivation of pulverulent raw materials and, if desired, subsequentshaping is preferred.

The catalysts of the invention can be used for the polymerization in theform of, for example, pellets, extrudates, spheres, rings or granules.Suitable shaped bodies and ways of producing them are described in DE10130782. Preference is given to using pellets, extrudates or spheres asshaped bodies. In the case of spheres, diameters of from 0.1 to 10 mm,preferably from 0.3 to 5 mm, are utilized. Pellets used preferably havediameters of from 1 to 5 mm and heights of from 1 to 3 mm. In the caseof extrudates, use is made of those having a diameter in the range 0.5-4mm, preferably 1-3 mm. The ratio of length to diameter of the preferredextrudates is usually from 20:1 to 0.5:1, preferably from 5:1 to 1:1.Apart from cylindrical extrudates, it is also possible to use, forexample, hollow extrudates, ribbed extrudates, star extrudates or otherextrudate shapes known to those skilled in the art.

As a pretreatment of the catalyst prior to use in the polymerizationreaction, it is possible to employ, for example, drying by means ofgases heated to from 80 to 200° C., preferably from 100 to 150° C., e.g.air or nitrogen.

Suitable telogens for the preparation of PTHF esters are carboxylicanhydrides or carboxylic anhydride/carboxylic acid mixtures. Amongthese, preference is given to aliphatic and aromatic polycarboxylicand/or monocarboxylic acids or their anhydrides which contain from 2 to12 carbon atoms. Examples of preferred telogens are acetic anhydride,propionic anhydride, succinic anhydride and maleic anhydride, in thepresence or absence of the corresponding acids. Particular preference isgiven to acetic anhydride as telogen.

The PTHF acetates formed when using the preferred telogens can beconverted into PTHF by various methods, for example by the methoddescribed in U.S. Pat. No. 4,460,796.

Other copolymers of THF can be prepared by the additional use of cyclicethers which are able to undergo ring-opening polymerization, preferablythree-, four- and five-membered rings such as 1,2-alkylene oxides, e.g.ethylene oxide or propylene oxide, oxetane, substituted oxetanes such as3,3-dimethyloxetane, and THF derivatives such as3-methyltetrahydrofuran, 3,3-dimethyltetrahydrofuran or3,4-dimethyltetrahydrofuran, particularly preferably3-methyltetrahydrofuran, as comonomers.

The telogen and, if desired, the comonomer is preferably introduced intothe polymerization as a solution in THF. Since the telogen leads tochain termination or to chain transfer in the polymerization, the meanmolecular weight of the polymer can be controlled via the amount oftelogen used. The more telogen present in the reaction mixture, thelower the mean molecular weight of the PTHF or the PTHF derivativeconcerned. PTHF, PTHF derivatives or THF copolymers having meanmolecular weights of from 250 to 10000 dalton can be prepared in atargeted fashion as a function of the telogen content of thepolymerization mixture. The process of the present invention ispreferably used to prepare PTHF, PTHF derivatives or THF copolymershaving mean molecular weights of from 500 to 5000 dalton, particularlypreferably from 650 to 4000 dalton.

The polymerization is generally carried out at from 0 to 80° C.,preferably at temperatures from 25° C. to the boiling point of THF. Thepressure employed is generally not critical to the result of thepolymerization, which is why the polymerization is generally carried outat atmospheric pressure or under the autogenous pressure of thepolymerization system. Exceptions to this are copolymerisation of THFwith volatile 1,2-alkylene oxides, which are advantageously carried outunder superatmospheric pressure. The pressure is usually from 0.1 to 20bar, preferably from 0.5 to 2 bar.

To avoid the formation of ether peroxides, the polymerization isadvantageously carried out under an inert gas atmosphere. As inertgases, it is possible to use, for example, nitrogen, carbon dioxide orthe noble gases; preference is given to using nitrogen.

The polymerization is particularly advantageously carried out under ahydrogen atmosphere. This embodiment gives a particularly low colornumber of the polymers formed. The hydrogen partial pressure can in thiscase be in the range from 0.1 to 50 bar. When the polymerization iscarried out in the presence of hydrogen, doping of the polymerizationcatalyst with transition metals or mixing the polymerization catalystwith a catalyst comprising transition metals makes it possible toachieve a further improvement in the color number. Transition metalsemployed are the elements of groups VIIA to VIIIA of the Periodic Table,for example ruthenium, rhenium, nickel, iron, cobalt, palladium and/orplatinum.

The process of the invention can be carried out batchwise orcontinuously, with the continuous mode of operation generally beingpreferred for economic reasons.

In the batchwise mode of operation, the reactants THF, the appropriatetelogen and/or, if desired, the comonomer and the catalyst are generallyreacted in a stirred vessel or loop reactor at the temperaturesindicated until the desired conversion of THF has been achieved.Depending on the amount of catalyst added, the reaction time can be from0.5 to 40 hours, preferably from 1 to 30 hours. The catalysts aregenerally added to the polymerization in an amount of from 1 to 90% byweight, preferably from 4 to 70% by weight and particularly preferablyfrom 8 to 60% by weight, based on the weight of the THF used.

In the continuous mode of operation, the reaction can be carried out inthe suspension or fixed-bed mode in conventional reactors or reactorassemblies suitable for continuous processes, for example in loopreactors or stirred reactors in the case of a suspension process and intube reactors or fixed-bed reactors in the case of a fixed-bed process,with preference being given to a fixed-bed process.

In the preferred fixed-bed mode of operation, the polymerization reactorcan be operated in the upflow mode, i.e. the reaction mixture is passedfrom the bottom upwards, or in the downflow mode, i.e. the reactionmixture is passed through the reactor from the top downward. The feedcomprising THF and telogen and/or comonomer is fed continuously into thepolymerization reactor, with the space velocity over the catalyst beingfrom 0.01 to 2.0 kg of THF/(1*h), preferably from 0.02 to 1.0 kg ofTHF/(1*h) and particularly preferably from 0.04 to 0.5 kg of THF/(1*h).

Furthermore, the polymerization reactor can be operated in a singlepass, i.e. without product recirculation, or in the circulation mode,i.e. part of the polymerization mixture leaving the reactor iscirculated. In the circulation mode, the ratio of recycle to feed isless than or equal to 150:1, preferably less than 100:1 and particularlypreferably less than 60:1.

The concentration of the carboxylic anhydride used as telogen in thefeed mixture fed into the polymerization reactor is from 0.03 to 30 mol%, preferably from 0.5 to 20 mol %, particularly preferably from 1 to 12mol %, based on the THF used.

If a carboxylic acid is additionally used, the molar ratio of thiscarboxylic anhydride in the feed is usually from 1:20 to 1:20000.

If additional comonomers are used, the molar ratio of these to THF inthe feed is usually from 0.1 to 60 mol %, preferably from 0.5 to 50 mol%, particularly preferably from 2 to 40 mol %.

If the polymerization is carried out in a suspension process, thework-up of the polymerization product mixture is carried out byseparating off the major part of the polymerization catalyst from thepolymerization mixture, for example by filtration, decantation orcentrifugation, and passing the resulting polymerization product mixtureto further work-up In the preferred fixed-bed mode, the polymerizationproduct mixture is subjected directly to the further work-up.

The work-up of the particularly preferred PTHF acetates or THF copolymeracetates can be carried out by methods known per se. For example,unreacted THF and, as appropriate, acetic anhydride, acetic acid andcomonomer are firstly separated off by distillation and the resultingPTHF acetate or THF copolymer acetate are transesterified with methanolin the presence of a basic catalyst to give PTHF or THF copolymer andmethyl acetate.

If desired, low molecular weight PTHF and/or tetrahydrofuran copolymerhaving a mean molecular weight of from 200 to 700 dalton cansubsequently be separated off by distillation. Low molecular weightcyclic oligomers are usually also separated off by distillation at thisstage. PTHF or THF copolymer having a mean molecular weight of from 650to 10,000 dalton remains as distillation residue.

The catalysts of the present invention can be regenerated after use in abatch or continuously operated PTHF process, for example by thermaltreatment as described in EP-A-0 535 515, and/or by washing the catalystwith aqueous and/or organic solvents.

The invention is illustrated by the examples below.

EXAMPLES

The specific surface area of the catalysts was measured by nitrogenadsorption (BET multipoint method) in accordance with DIN 66131.

The mesoporosity and the mesopore distribution of the catalysts wasdetermined by nitrogen desorption (BJH N₂ isotherm method) in accordancewith DIN 66134. The data were evaluated in the pore diameter range from2 nm to 200 nm. References to maxima in the pore radius distribution arebased on a logarithmic plot of the pore radius.

The porosity of shaped catalyst bodies was determined by mercuryintrusion in accordance with DIN 66133. The data were evaluated in thepore radius range from 20A to 10 μm.

The methylene blue adsorption was carried out by the spot analysismethod as described in the leaflet “Bindemittelprüfung/Prüfung vonBindetonen” of the Vereins Deutscher Giessereifachleute (VDG) (draft P69 E of June 1998).

The analysis of the concentration of acid centers in the catalyst wascarried out by titration in aqueous solution. For this purpose, from 1to 2 g of catalyst were weighed out, suspended in 100 ml of 0.1 mol/lsodium nitrate solution and titrated with 0.1 mol/l sodium hydroxidesolution while stirring continually. Between two addition steps, it wasnecessary to wait for a sufficient time for a constant pH to beestablished (about 5 min). The concentration of the acid centers and theassociated pK_(a) values can be read off from the plot of the pH versusthe amount of sodium hydroxide added. In the examples, the acid centerdensity reported is that to the equivalence point which can be observedin the titration curve at a pH of about 5-7.

To determine the proportion of alkali-soluble silicon dioxide, 1 g ofdried catalyst powder were admixed with 100 ml of sodium carbonatesolution (2% by weight) and boiled for 10 minutes. The supernatantsolution was decanted off and fines were filtered off by means of asuction filter. The residue was boiled twice more with 25 ml of sodiumcarbonate solution for 2 minutes and once again decanted and filtered.The residue was finally placed on the suction filter and washed with hotsodium carbonate solution. The combined filtrates were made up to 0.5 land analyzed for their sodium silicate content.

For the analysis of the acid conversion, the proton concentrationremaining in the suspension after the activation was determined bytitration and expressed as a ratio to the amount of acid used. The endpoint was determined by conductivity measurements (conductometrically),since pH titrations measure not only the remaining activation acid butalso acidic cations and are therefore unsuitable for this purpose.

The determination of the exchangeable cations in the raw clay wascarried out by ion exchange with ammonium. For this purpose, 30 g ofdried clay were refluxed with 300 g of NH₄Cl solution (2 mol/kg) for 2hours. After a further period of 16 hours, the clay was separated offand washed with 0.5 l of deionized water. The mother liquor and thewashings were made up to 1 l and the cations present (Na, K, Mg, Ca)were determined by elemental analysis. The ratio of the alkali metalions to alkaline earth metal ions (total Na+K or Mg+Ca in equivalents)indicates whether the clay is a sodium bentonite (ratio of about 1 orabove) or a calcium bentonite (ratio significantly less than 1).

The productivity of the catalyst (g of PTHF diacetate/g of catalyst *h)in powder form was determined as follows:

200 g of tetrahydrofuran and 20 g of acetic anhydride were placed in a250 ml flask and heated to 50° C. While stirring vigorously, 2-3 g of afinely powdered catalyst which had been dried at 120° C. and hadparticle sizes of at least 100 μm were added and the reaction mixturewas stirred vigorously at 50° C. for a further period. 20 ml samples ofthe reaction mixture were taken after 45 minutes, 2 hours, 4 hours and 6hours and promptly separated from the catalyst powder by filtration. Thesamples which had been freed of catalyst were analyzed for PTHFdiacetate, e.g. by separating off the low boilers by distillation andweighing the PTHF diacetate. The productivity of the catalyst powder wasdetermined by extrapolation of the time dependence of PTHF diacetateformation.

The examples of productivity determination described can easily beadapted in respect of type and amount of telogens and/or comonomers,temperature, pressure, etc., to the respective use conditions of thecatalyst. Variation of catalyst amounts and reaction times additionallyenables the experiments to be easily modified to give various highinitial productivities.

Example 1

A raw bentonite from Wyoming displayed the following characteristics:methylene blue adsorption: 310 mg/g; ratio of equivalents ofexchangeable alkali metali cations to alkaline earth metal cations: 1.1;BET surface area: 34 m²/g. Composition data found were: 9% by weight ofmoisture; 15% by weight loss on ignition; 25.3% by weight of Si; 9.5% byweight of Al, 2.5% by weight of Fe; 1.5% by weight of Na; 1.2% by weightof Mg and 1.0% by weight of Ca.

90 g of this raw bentonite were suspended in 300 g of hydrochloric acidhaving a concentration of 0.75 mol/kg and the suspension was stirred at100° C. for 24 hours. A further 53 g of concentrated hydrochloric acid(32% strength by weight) were subsequently added and the mixture wasstirred for a further 24 hours at 100° C. During the total activationtime, 7.3 mmol of hydrochloric acid were consumed per g of raw clay.

After suction filtration, washing with deionized water and drying at120° C., the catalyst displays an acid center density of 0.37 mmol/g, aBET surface area of 227 m²/g and a productivity in the polymerizationtest of 13 g/g*h.

Example 2

A raw bentonite of Indian origin displayed the followingcharacteristics: methylene blue adsorption: 404 mg/g; ratio ofequivalentts of exchangeable alkali metal cations to alkaline earthmetal cations: 1.1; BET surface area: 116 m²/g. Composition data foundwere: 14% by weight of moisture; 20% by weight loss on ignition; 22.4%by weight of Si; 8.9% by weight of Al, 6.8% by weight of Fe; 1.3% byweight of Na; 1.1% by weight of Mg and 0.1-1% by weight each of Ca, Tiand Mn.

20 kg of this raw bentonite were suspended in 66.6 kg of sulfuric acidhaving a concentration of 0.75 mol/kg and the suspension was stirredslowly at 95-98° C. for 24 hours. 16 kg of half-concentrated sulfuricacid (50% strength by weight) were subsequently added and stirring wascontinued at 95-98° C. for a further 24 hours. The suspension wasdiluted with 70 l of deionized water, filtered in a filter press and thesolid was washed with about 700 l of deionized water. The filter cakewas subsequently resuspended in 32 l of dilute nitric acid (1% strengthby weight), filtered again and washed with 900 l of deionized water.Drying gave 12.5 kg of catalyst powder having an acid center density of0.40 mol/g and a productivity in the polymerization test of 15 g/g*h.The catalyst contained 70.6% by weight of SiO₂; 10.8% by weight of Al₂O₃and 4.6% by weight of Fe₂O₃. The catalyst contained 58% by weight ofalkali-soluble SiO₂. The BET surface area is 403 m²/g and the BJH-N₂pore volume is 0.59 cm³/g, of which 0.33 cm³/g is made up by pores inthe diameter range from 5 to 50 nm.

Example 3

210 g of the raw bentonite from Example 2 were suspended in 700 g ofhydrochloric acid having a concentration of 1.0 mol/kg and thesuspension was stirred at 100° C. for 24 hours. A further 105 g ofconcentrated hydrochloric acid (32% strength by weight) weresubsequently added and the mixture was stirred for a further 24 hours at100° C. During the total activation time, 8.2 mmol of hydrochloric acidwere consumed per g of raw clay.

After suction filtration, washing with deionized water and drying at120° C., the catalyst displays an acid center density of 0.44 mmol/g, aBET surface area of 370 m²/g and a productivity in the polymerizationtest of 15 g/g*h.

Example 4

35 g of the raw bentonite from Example 2 were suspended in 700 ml ofhydrochloric acid having a concentration of 0.6 mol/l and the suspensionwas stirred at 100° C. for 72 hours. During the activation, about 8 mmolof hydrochloric acid are consumed per g of raw clay. After suctionfiltration, washing with deionized water and drying at 120° C., thecatalyst displays an acid center density of 0.38 mmol/g, a BET surfacearea of 357 m²/g and a productivity in the polymerization test of 16g/g*h.

Example 5

A raw bentonite of Bavarian origin which has been preactivated usingsodium carbonate displays the following characteristics: methylene blueadsorption: 260 mg/g; ratio of equivalents of exchangeable alkali metalications to alkaline earth metal cations: 0.9; BET surface area: 84 m²/g.Composition data found were: 9% by weight of moisture; 14% by weightloss on ignition; 25.4% by weight of Si; 9.0% by weight of Al, 3.5% byweight of Fe; 1.7% by weight of Mg; 1.2% by weight of Na; 1.2% by weightof Ca and 0.3% by weight of Ti.

90 g of this raw bentonite were suspended in 300 g of hydrochloric acidhaving a concentration of 0.75 mol/l and the suspension was stirred at100° C. for 24 hours. A further 63.6 g of concentrated hydrochloric acid(32% strength by weight) were subsequently added and the mixture wasstirred for a further 24 hours at 100° C. During the total activationtime, 8.2 mmol of hydrochloric acid were consumed per g of raw clay.

After suction filtration, washing with deionized water and drying at120° C., the catalyst displays an acid center density of 0.26 mmol/g, aBET surface area of 312 m²/g and a productivity in the polymerizationtest of 10 g/g*h.

Comparative Example I

150 g of the raw bentonite from Example 2 were suspended in 500 g ofhydrochloric acid having a concentration of 3 mol/kg and the suspensionwas stirred for one hour. 0.4 mmol of acid were reacted per gram of clayused. The activated clay was filtered off with suction, washed free ofchloride and dried at 120° C. The catalyst displays an acid centerdensity of 0.55 mmol/g and a BET surface area of 137 m2/g. Theproductivity of the catalyst powder in the polymerization test was 4g/g*h.

Comparative Example II

150 g of a raw bentonite corresponding to Example 2 were suspended in500 g of hydrochloric acid having a concentration of 4.5 mol/kg and thesuspension was stirred at 100° C. for 72 hours. The activated clay wasfiltered off with suction, washed free of chloride and dried at 120° C.The catalyst displays an acid center density of about 0.02 mmol/g. Theproductivity of the catalyst powder in the polymerization test was 0.5g/g*h.

Example 6 Continuous Polymerization

300 g of a catalyst which had been acid-activated using a methodanalogous to Example 3 were kneaded with 275 ml of water in a laboratorykneader for 10 minutes, subsequently extruded to give extrudates havinga diameter of 2.5 mm, dried and finally calcined at 350° C. The catalystdisplays a mercury porosity of 0.45 cm³/g.

In a laboratory apparatus, a mixture of THF and acetic anhydride (6.9%based on the total feed) was passed at 45° C. under protective gas overthis catalyst which had been predried at 140° C. and was arranged as afixed bed in a 250 ml reactor (internal diameter: 40 mm). The spacevelocity over the catalyst was 0.2 kg of feed/(1 of cat.*h). The reactorwas operated with product recirculation (about 1 l/h). To work-up thePTHF diacetate, the reaction mixture obtained was freed of unreacted THFand acetic anhydride by distillation. The degree of evaporation wasabout 56%, and the molecular weight of the PTHF diacetate (Mn) was850-900 g/mol.

1. A solid, acid catalyst for the preparation of polytetrahydrofuran,polytetrahydrofuran copolymers, and diesters or monoesters of thesepolymers, by polymerization of tetrahydrofuran in the presence of atleast one telogen and/or comonomer, said catalyst comprising a claymaterial which comprises at least 20% by weight of SiO₂ and at least onefurther oxide of an element selected from the group consisting of Al, Feand the elements of groups III A to VII A of the Periodic Table, andwherein said catalyst further comprises a proportion of alkali-solublesilicon dioxide of from 20 to 85% by weight, where the catalyst has beencalcined at from 150 to 800° C. and has an N₂ pore volume of at least0.35 cm³/g for pore diameters in the range from 2 to 200 nm, with atleast 0.2 cm³/g of this N₂ pore volume being made up by pores havingdiameters in the range 5-50 nm and the mean BJH pore diameter (4V/A) ofthe pores in the range from 2 to 200 nm being from 2.0 to 10.0 nm, has aBET surface area of at least 160 m²/g and has an acid center density ofat least 0.25 mmol/g for pK_(a) values of from 1 to
 6. 2. A catalyst asclaimed in claim 1, wherein the clay material has a methylene blue valueof at least 250 mg/g.
 3. A catalyst as claimed in claim 1, wherein theclay mineral is a sodium bentonite.
 4. A catalyst as claimed in claim 2,wherein the clay mineral is a sodium bentonite.